Wurtzite-structure AlGaInBN has very large direct band gaps of 0.7 eV to 6.2 eV, and can form quantum well structures and two-dimensional electron gases by forming heterojunctions. Further, the breakdown field of GaN is 3.3×106 V/cm, which is approximately eight times larger than that of GaAs, and GaN has a high saturation electron velocity. With these excellent characteristics, wurtzite-structure AlGaInBN has been applied and commercialized in the following three fields:
(1) The first ones are light-emitting diodes (LEDs) in the blue-to-green range of the visible region constructed using wurtzite-structure AlGaInBN. The LEDs have features such as low power consumption, long life, and small size, and have been widely commercialized in traffic signals, electronic signage, backlights for displays, LED displays, various kinds of lighting, exposure light sources, and the like. Moreover, semiconductor lasers (LDs) constructed using wurtzite-structure AlGaInBN have smaller sizes, lower power consumption, and lower manufacturing costs than other lasers, and therefore have been widely commercialized in video game consoles and light sources for next-generation optical discs.
(2) The second ones are high-frequency, high-output devices constructed using wurtzite-structure AlGaInBN. For example, AlGaN/GaN high-electron-mobility transistors (HEMTs) have two-dimensional electron gases with high carrier concentrations and high saturation electron velocities and high breakdown fields, and are therefore being commercialized as devices for mobile phone base stations by being used as millimeter-band, high-frequency, high-output transistors.
(3) The third ones are high-efficiency solar cells constructed using wurtzite-structure AlGaInBN. Nitride semiconductors are wide band-gap semiconductors, and can absorb sunlight in almost the whole range. Moreover, by forming heterostructures, very high-efficiency solar cells may be realized.
Heretofore, wurtzite-structure AlGaInBN has been grown on a sapphire substrate. Sapphire substrates are relatively inexpensive, may be large-area substrates, and are excellent in crystallinity and electrically insulative, thus being widely used in application to the above-described devices. However, since there are a large lattice mismatch and a thermal expansion coefficient difference between wurtzite-structure AlGaInBN and a sapphire substrate, it has been difficult to grow a single-crystal wurtzite-structure nitride semiconductor such as GaN, AlN, or AlGaN.
To solve this problem, it has already been proposed that a buffer layer is grown on a sapphire substrate, followed by the growth of wurtzite-structure AlGaInBN on the buffer layer. For the buffer layer, the following growing methods have been proposed: a method (referred to as “LT-AlN”) in which an AlN layer is grown at a low temperature, a method (referred to as “LT-GaN”) in which a GaN layer is grown at a low temperature, and a method (referred to as “AlON”) in which an Al2O3/AlON/AlN/Al2O3 multilayered film is grown by ECR plasma sputtering.
In LT-AlN, a thin amorphous AlN layer is formed as a buffer layer at a low temperature of approximately 500° C. using a metalorganic chemical vapor deposition (MOCVD) system, the buffer layer is annealed at a high temperature of approximately 1000° C., and a nitride semiconductor such as GaN is grown on the annealed AlN layer (see Non-Patent Literature 1). It has been reported that LT-AlN allows single-crystal, crack-free, high-quality GaN to grow on a sapphire substrate.
In LT-GaN, a thin amorphous GaN layer is formed as a buffer layer at a low temperature of approximately 500° C. using an MOCVD system as in LT-AlN, the buffer layer is annealed at a high temperature of approximately 1000° C., and a nitride semiconductor such as GaN is grown on the annealed GaN layer (see Non-Patent Literature 2). It has been reported that LT-GaN allows single-crystal, high-mobility, high-quality GaN to grow on a sapphire substrate by LT-GaN.
In AlON, a multilayered film including an Al2O3 layer, a graded-composition AlON layer, an AlN layer, and an Al2O3 layer is grown as a buffer layer on a sapphire substrate by ECR plasma sputtering, and GaN is grown on the buffer layer by MOCVD (see Non-Patent Literature 3). It has been reported that planar, high-quality GaN can be grown by this technique.
Moreover, for realizing a ubiquitous society in which light-emitting devices such as LEDs and LDs constructed using wurtzite-structure AlGaInBN can be used anytime and anywhere, it is necessary to grow wurtzite-structure AlGaInBN on a larger-area or more inexpensive, bendable (flexible) substrate. However, growth on such a substrate is difficult. Moreover, in the case where a high-frequency, high-output device such as described above is grown on a sapphire substrate or a silicon carbide substrate, such a substrate has a heat dissipation problem because of the low thermal conductivity thereof. This heat dissipation problem considerably limits device characteristics. Growth on a substrate, e.g., a copper substrate, having a high thermal conductivity is desired. However, growth on such a substrate is difficult. Further, solar cells are expected to be used outdoors and at large-area and bendable places, and are therefore desired to be grown on a large-area or inexpensive, bendable substrate, a transparent plastic or glass substrate, or the like. However, conventional techniques cannot grow solar cells on such a substrate.
To solve such problems, it has been proposed that a wurtzite-structure AlGaInBN thin film grown on a sapphire substrate or a silicon carbide substrate is separated from the sapphire substrate or the silicon carbide substrate to be transferred to a second substrate suitable for the application, and this is being researched.
A wurtzite-structure AlGaInBN thin film can be grown on a sapphire substrate or a silicon carbide substrate by vapor phase epitaxy (VPE) or MOCVD. To separate the wurtzite-structure AlGaInBN thin film grown by VPE or MOCVD from the sapphire substrate or the silicon carbide substrate and transfer the wurtzite-structure AlGaInBN thin film to a second substrate, there are two methods proposed. One is to irradiate a wurtzite-structure AlGaInBN thin film grown on a low temperature-grown GaN buffer layer on a sapphire substrate with, for example, an excimer laser having a wavelength of 248 nm to melt GaN at the interface with the sapphire substrate, thus separating the wurtzite-structure AlGaInBN thin film from the sapphire substrate. The separated wurtzite-structure AlGaInBN thin film is transferred to a second substrate. This method is called “laser lift-off” (see Non-Patent Literature 4). The other is to grow a chemically etchable chromium nitride (CrN) or zinc oxide (ZnO) buffer layer on a sapphire substrate, grow a wurtzite-structure AlGaInBN thin film on the buffer layer, and etch the buffer layer existing at the interface by chemical etching after the growth, thus separating the wurtzite-structure AlGaInBN thin film from the sapphire substrate. The separated wurtzite-structure AlGaInBN thin film is transferred to a second substrate. This method is called “chemical lift-off” (see Non-Patent Literature 5 and 6).
In this method, CrN is grown on the sapphire substrate by sputtering, and a nitride semiconductor such as GaN is grown on the CrN by MOCVD (Non-Patent Literature 5). By immersing the sample in a perchloric acid-based etchant for CrN, the CrN sacrificial layer is chemically etched, and the nitride semiconductor thin film made of GaN or the like is separated from the sapphire substrate.
Alternatively, ZnO is grown on the sapphire substrate by pulsed laser deposition, and a nitride semiconductor such as GaN is grown on the ZnO by MOCVD (Non-Patent Literature 6). By immersing the sample in diluted hydrogen chloride, the ZnO sacrificial layer is chemically etched, and the nitride semiconductor thin film made of GaN or the like is separated from the sapphire substrate.
Further, wurtzite AlGaInBN is also grown on a silicon (Si) substrate. Si substrates are inexpensive, may be large-area substrates, and are excellent in crystallinity, thus being used in the above-described application to LED devices. However, an LED on a Si substrate has the following problem: light emitted from a multiple-quantum-well structure as a light-emitting layer is absorbed by the Si substrate, and, as a result, the light extraction efficiency of the LED on the Si substrate significantly decreases. Further, in the LED on the Si substrate, for the reduction of meltback etching and a lattice mismatch between the Si substrate and GaN, a wurtzite AlN layer needs to be grown as a buffer layer on the Si substrate. However, it is generally difficult to obtain an n-type AlN layer by doping the wurtzite AlN layer with Si due to self-compensation effect. Thus, there is the problem that it is difficult to grow vertical LEDs, which will be indispensable for high-output LEDs in the future, on Si substrates.
To solve this problem, it has been proposed that a wurtzite AlGaInBN thin film is grown on a SOI (Si-on-insulator) substrate. SiO2 and a Si sacrificial layer formed thereon are chemically etched and mechanically polished after growth. As a result, the wurtzite AlGaInBN thin film can be separated from the SOI substrate and transferred to a metal substrate with high reflectance.
In this method, SOI is grown in a Si substrate by the SIMOX (Separated by Implanted Oxygen) process, and a nitride semiconductor such as GaN is grown on the SOI by MOCVD (Non-Patent Literature 7). The sample is mechanically polished and immersed in a potassium hydroxide solution, whereby SiO2 and a Si sacrificial layer formed thereon are chemically etched. Thus, the nitride semiconductor thin film made of GaN or the like can be transferred from the Si substrate to a metal substrate with high reflectance.